Hydrostatically driven vehicles typically include a hydraulic pump driven by an engine or motor. The hydraulic pump propels a flow of fluid to one or more actuators, typically hydraulic motors, connected to wheels or other driving features of the vehicle. The flow of fluid from the pump passes through each actuator, causing the vehicle to move along at a travel speed. An operator adjusting a control input, for example, a lever, pedal, or any other appropriate device controls motion of the vehicle. When the operator displaces the control input, a signal is generated by a displacement sensor integrated with the control input or, alternatively, by displacement of a mechanical linkage. The signal is conveyed to a controller associated with the vehicle, where it is interpreted and an appropriate command is issued to an actuator associated with the hydraulic pump, the actuator being arranged to move a control arm of the pump operating to change the displacement of the pump. Alternatively, the control input may be mechanically connected to the pump, for example, by cable, which causes the control arm of the pump to move in response to displacement of the control input.
Displacement of the control arm of the pump causes a change in the pump's displacement by changing the angle of operation of a swashplate within the pump and, accordingly, a change in the pressure and flow rate of fluid propelled through the pump. Modulation of the flow rate of fluid through the pump controls, for example, the rate of rotation of hydraulic motors driving the wheels of the vehicle and, therefore, the travel speed of the vehicle. Additional systems may be available for control of the travel speed of the vehicle, for example, braking systems or transmissions may be used to decelerate the vehicle when the operator so desires.
Even though these types of control have been effective in the past, they are inadequate in preventing uncommanded motion of the vehicle under some circumstances, for example, when heavy vehicles, such as asphalt compactors, are operating on graded surfaces or inclines. Preventing uncommanded motion of the vehicle is desirable not only for safety concerns, but also because operation of the vehicle at a faster or slower than desired pace will directly affect operating parameters, such as, the efficiency and rate of compaction of material.